Communication systems, such as Public Switched Telephone Networks (PSTN), cellular wireless networks (e.g. GSM, LTE, UMTS, CDMA), IP-telephony (Voice over IP, VoIP), offers the user emergency call services via an Application Programming Interface (API). In this context, the modem is referred to as the module that provides the API and interfaces the communication system. Examples of modems are:
PSTN modems, which is connected to the PSTN network/switch,
GSM, LTE, or other modems for wireless communication systems, which interface the GSM, LTE air interface,
VoIP modems, which includes both the application (usually in software) and the IP provider, e.g. LAN, WLAN, Bluetooth, cellular network.
An application/user interface may have access to none, one, or several modems of each type of the ones listed above. As an example, a mobile telephone or a tablet offers emergency call services via several cellular networks, either one at a time using one SIM (e.g. GSM, UMTS, LTE, one at a time) or several SIMs at a time using dual/multiple SIMs. Besides that, a mobile telephone offers emergency call service via one or several VoIP modems, with a flexible IP provider; via the cellular networks or via WIFI, USB or Bluetooth. Another example is a portable or stationary computer, which may offer emergency call services via one or several VoIP modems.
The SIM card is a physical hardware of integrated circuit which is inserted in the mobile phone, and is provided in different sizes such as ordinary SIM, mini SIM, micro SIM, and nano SIM. The SIM comprises among other things the International Subscriber Identity (IMSI) and a related key which is used to identify and authenticate subscribers. The modem on the other hand is usually the software together with hardware which handles the air interface with the RAN/RAT and in this respect uses the SIM card in that purpose as described above.
When a mobile phone (or cellular phone, cell phone, etc.) has more than one Subscriber Identity Module (SIM), from here on referred to as Multiple SIM phone, one SIM is considered the primary SIM and the rest of the SIMs in the Multiple SIM phone are considered as secondary SIMs. Generally the user of the mobile phone selects which SIM that is the primary SIM depending on e.g. the cost plan, data rate requirements, etc.
The SIM may be connected to different mobile operators or different Radio Access Networks (RANs) of a wireless communication system. The RAN may support different Radio Access Technologies (RATs), such as E-UTRA (LTE), GSM, which provide different coverage, data rates, latency, etc.
According to prior art, when an emergency call (e.g. number 112, 911, etc. depending on country) is placed by the user on a Multiple SIM phone, mentioned phone tries over and over again to establish the emergency call on the primary SIM. Alternatively, the Multiple SIM phone tries to establish the emergency call on a secondary SIM, potentially failing resulting in a try on the next secondary SIM. The emergency call is placed on the primary SIM of a Multiple SIM phone, or if the primary SIM is out of coverage or establishment fails, on a secondary SIM in a predefined prioritization order, usually depending on the cost plan. In any case, the establishment of the emergency call is delayed for every non successful establishment. Further, in emergency situations there is a risk that the mobile phone tries to establish the emergency call on the primary SIM which is not possible due to overloaded network, low signal quality, or other issues. Moreover, even if the emergency call is established on the primary SIM, the emergency call may suffer from unacceptable voice quality or being dropped due to deteriorating signal levels, etc.
FIG. 1 shows a simplified system overview of a Multiple SIM phone supporting N number of modems indexed n (M—1, SIM—2, . . . , SIM_N), where n and N are positive integers. Further, each SIM is mapped to its associated modem. Assume further that modem M—2 is the primary modem on the Multiple SIM phone. The Emergency Call Coordinator, which can be a module in the phone, co-ordinates the signalling related to Emergency Call Establishment.
FIG. 2 illustrates a prior art method in a communication device having multiple communication modems. The sequence of Emergency Call Establishment is established as:                The user places the emergency call by dialing a country specific emergency number, e.g. 911 or 112, via a User Interface (UI) of the Multiple SIM phone;        The Emergency Call Establishment is indicated to modem M—2 associated with the primary SIM—2; and        Modem M—2 initiates the emergency call establishment procedure.        